The present invention two-stroke gasoline engine is of the scavenging type wherein through a scavenging port, fresh pre-mixed fuel air mixture flows into the cylinder and flushes the same, and the exhaust gas is scavenged through an exhaust port when the exhaust port is opened. Two shortcomings exist in such scavenging process as follows:
First, during scavenging process, the fresh fuel air mixture serving as scavenging gas flow often directly escapes through the exhaust port, causing loss of short-circuit. When the engine works under high loading condition, such loss appears more serious.
Second, during scavenging process, the burned exhaust gas is not totally scavenged and mixes with the fresh fuel air mixture. After the scavenging process ends, part of the exhaust gas remains in the cylinder, and when the engine works under low loading condition, because the scavenging gas flow is weaker, the scavenging efficiency is correspondingly lower, and consequently, the ratio of remaining exhaust gas to fresh fuel air mixture is too high. Such too high ratio is extremely apt to cause misfire of engine and lower the combustion efficiency.
The above two shortcomings are the major reasons why great amount of hydrocarbon is contained in the exhaust gas exhausted from two-stroke gasoline engine.